Add .bazelversion file

Add a .bazelversion file with a deliberately old version. This will make
it easy to test a roller of it.

Bug: b/356645484
Change-Id: I019018e74fa8ecee9845f7de275a456cb95ef382
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/226895
Reviewed-by: Ted Pudlik <tpudlik@google.com>
Commit-Queue: Auto-Submit <auto-submit@pigweed-service-accounts.iam.gserviceaccount.com>
Pigweed-Auto-Submit: Rob Mohr <mohrr@google.com>
Lint: Lint 🤖 <android-build-ayeaye@system.gserviceaccount.com>
Presubmit-Verified: CQ Bot Account <pigweed-scoped@luci-project-accounts.iam.gserviceaccount.com>
diff --git a/.bazelversion b/.bazelversion
new file mode 100644
index 0000000..a8a1887
--- /dev/null
+++ b/.bazelversion
@@ -0,0 +1 @@
+7.1.2